Chemical sensing is a critical process in a large number of everyday household, manufacturing, health care, industrial, military, and scientific processes. A chemical sensor that can indicate the presence of a chemical of interest is useful to provide warnings, such as to indicate an unacceptable level of carbon monoxide or to provide a warning regarding the presence of an explosive vapor or a chemical warfare agent. Similarly, chemical sensors can provide information on the presence or absence of a particular chemical in a process control scheme. The presence or absence of a gas can provide feedback used to control a wide range of industrial processes. In the area of scientific research, many instruments including, for example, chromatography instruments benefit from sensitive chemical detectors.
Sensitivity is a critical aspect of chemical sensors. The more sensitive a sensor is, the lower level of chemical agent that it can detect. Accordingly, there is great interest in producing highly sensitive chemical sensors. Early warning regarding levels of sensed chemicals, faster control of processes responsive to particular levels of sensed chemicals, and better detection in difficult environments are achieved as sensitivity increases. Some particular example applications of interest in the art will now be discussed.
One application of interest is the detection of ultra-trace amounts of explosives and explosive-related analytes. Such detection is of critical importance in detecting explosives in a number of civilian and military or security applications, e.g., mine fields, military bases, remediation sites, and urban transportation areas. Low-cost and portability have clear additional advantages to such sensor applications.
In security applications, chemical sensors are preferable to other detection devices, such as metal detectors, because metal detectors frequently fail to detect explosives, such as those in the case of the plastic casing of modern land mines. Similarly, trained dogs can be both expensive and difficult to maintain in many desired applications. Other detection methods, such as gas chromatography coupled with a mass spectrometer, surface-enhanced Raman Spectroscopy, nuclear quadrupole resonance, energy-dispersive X-ray diffraction, neutron activation analysis and electron capture detection are highly selective, but are expensive and not easily adapted to a small, low-power package for broad distribution.
Vapor-phase detection of peroxides is critically important for many health care, military and industrial safety applications. Hydrogen peroxide (H2O2) is a common oxidant, used industrially for paper bleaching and specialty chemical manufacture. It is also used in medical facilities as a chemical disinfectant. H2O2 is quite toxic; in the vapor phase 75 ppm (which may be present from the vapor of 30% H2O2 in water), is immediately hazardous to health, and the OSHA permissible exposure limit (PEL) for an 8 hour period is 1 ppm. Even those levels can be unsuitable for hospitalized patients, and H2O2 is an important sterilization tool for many hospital applications, such as surgical instrument sterilization, drug container sterilization, and room sterilization. Other techniques are unsatisfactory from a sterilization perspective, but use of H2O2 creates a risk of residues that are harmful at OSHA or even lower levels. Due to its widespread use and toxicity, vapor-phase monitoring of hydrogen peroxide is a necessity.
Concentrated hydrogen peroxide solutions are also precursor materials for organic peroxide based explosives, which are commonly used by terrorists in improvised explosive devices, making detection of both organic peroxides and hydrogen peroxide important to military and law-enforcement agencies.
Peroxide-based explosives, such as triacetone triperoxide (TATP), have seen marked increase in use over the past ten years. TATP in particular has been implicated in the London bombings of Jul. 7, 2005, as well as in the attempted airplane bombing by Richard Reid in December 2001. It has been widely used in improvised explosive devices by terrorists in countries (e.g. Israel) where the sale of high explosives is carefully monitored. TATP is a volatile compound (vapor pressure 5.2*10−2 Torr under ambient conditions) susceptible to detonation from heat, friction and shock, and it is primarily used in illegal activities rather than for military applications.
There are a variety of detection methods currently being developed and used for the detection of hydrogen peroxide and organic peroxides. These technologies include chromatographic/spectroscopic platforms, mass spectrometric systems, amperometric sensors, and fluorescent chemical assays. The chromatographic systems include gas chromatography and liquid chromatography (LC and HPLC) interfaced with FTIR and fluorescence detection. These systems offer high sensitivity and selectivity, but suffer from such drawbacks as relative size and lack of portability, high power demands, and sophisticated computational requirements. Mass spectrometry is widely used and suffers from similar drawbacks. Amperometric sensors and fluorescent chemical assays are extremely sensitive, and can be used to detect trace amounts of peroxides, but these methods only detect liquid/solid peroxides and may be susceptible to false positives.
Cross-reactive sensor array reactive sensor arrays have been shown to discriminate between chemicals in complex mixtures, and under favorable conditions can be used for field detection to identify compounds of interest in a complex background. See, Albert et al., “Cross-reactive Chemical Sensor Arrays”, Chem. Rev. 2000, 100, 2595-2626. Metallophthalocyanines (MPcs) are compounds that have been used in chemiresistive sensors. See, Gould, R. D. Structure and Electrical Conduction Properties of Phthalocyanine Thin Films, Coord. Chem. Rev. 1996, 156, 237-274. The responses of MPc films to various oxidizing gases at constant direct current (DC) bias has been studied, resulting in a large body of literature on the influence of O2, NO2, O3, and H2O analytes on MPc resistive sensors. See, Snow, A. W. et al., “Phthalocyanine Films in Chemical Sensors;” Phthalocyanines: Properties and Applications; Lever, A. B. P., Ed; John Wiley and Sons: New York, 1989; Vol. 1, p. 341; Wright, J. D., et al, Gas Adsorption on Phthalocyanines and its Effects on Electrical Properties, Prog. Surf. Sci. 1989, 31, 1-60.
Metallophthalocyanines are readily prepared in a simple, one-pot synthesis of the appropriately substituted phthalonitrile and metal salt of interest by refluxing in a high-boiling alcohol with a catalytic amount of strong base for several hours Metallophthalocyanines have been synthesized with nearly every metal in the periodic table. The phthalocyanines studied most are those of the late first row transition metals, such as iron, cobalt, nickel, copper, and zinc. Phthalocyanines are generally p-type semiconductors, with holes being the active charge carriers. N-type metallophthalocyanines semiconductors are also known. Perfluorinated metallophthalocyanines, where the 16 outer protons are replaced by fluorine atoms are n-type semiconductors. Metallophthalocyanines films have been shown to exhibit ohmic conductivity and space charge limited conductivity.
Conductivity in phthalocyanines films is influenced strongly by atmospheric “dopants,” primarily O2. When phthalocyanines thin films are exposed to O2, the films become doped and the conductivity increases dramatically. This air-induced conductivity has been attributed to different mechanisms. Resistive sensing studies with p-type phthalocyanines thin films have focused primarily on their interaction with oxidizing gases, such as ozone and NOx. See, e.g., Lee, Y. L. et al, “Effects of Substrate Temperature on the Film Characteristics and Gas-Sensing Properties of Copper Phthalocyanine Films, Appl. Surf. Sci. 2001, 172, 191-199; Liu, C. J et al., Response Characteristics of Lead Phthalocyanine Gas Sensor: Effect of Operating Temperature and Postdeposition Annealing, Journal of Vacuum Science & Technology A-Vacuum Surfaces and Films, 1996, 14(3), 753-756; Sadaoka, Y, et al., “Fast NO2 Detection at Room Temperature with Optimized Lead Phthlocyanine Thin-Film Structures”, Sens. Actuators, B 1990, 148-153. Phthalocyanines films are easily oxidized by NOx, forming charge transfer complexes, which inject holes and increase film currents. The interaction of phthalocyanines films with reducing gases, such as NH3, has the opposite effect. Decreased current upon analyte binding to these films has been attributed to electron donation from the reducing gas to trap charge carriers. Bohrer, F. I. et al, Gas Sesning Mechanism in Chemresitive Cobalt and Metal-Free Phthalocyanine Thin Films”, J. Am. Chem. Soc. 2007, 129, 5640-5646.
H2O2 interactions with CoPc have been extensively studied in solution phase amperometric apparatuses. It has been reported that FePc and CoPc can electrochemically catalyze both oxidation and reduction of H2O2. Electrooxidation of H2O2 has been found to occur over electron-deficient films, which agrees with the use of hole-conducting, O2 doped FePc and CoPc films in the present invention. From the drastic current loss upon exposure of FePc and CoPc to H2O2 it is apparent that chemisorption and subsequent oxidation of H2O2 (and concurrent reduction of the MPc film) is occurring. The irreversibility of the current reduction in the FePc films suggests that some further reaction within the film is occurring, such as the formation of the μ-oxo dimer of FePc. The mechanism of interaction of H2O2 with CuPc and H2Pc is quite different. These films are oxidized by the dosing of H2O2′ which may arise from homolytic cleavage of H2O2 to hydroxyl radicals, and electron extraction from the organic ring of the phthalocyanine.
The interaction of di-tert-butyl peroxide with MPc films has not been reported in the literature. H2Pc and CoPc show minimal responses when dosed with di-tert-butyl peroxide, suggesting that simple physisorption is occurring, rather than redox chemistry. In contrast, CuPc is oxidized by di-tert-butyl peroxide, likely by a similar mechanism to H2O2: homolytic cleavage of the peroxide bond followed by electron extraction from the organic ligand. FePc is strongly oxidized by di-tert-butyl peroxide in a similar mechanism to that of CuPc.
In contrast to the redox reactions explained above, the interactions of phthalocyanines with common interferent vapors such as water, volatile organic compounds (VOCs), or other electron donors, involve coordination of the molecules to the metallophthalocyanine metal center and hole destruction in the semiconductor film by oxygen displacement, as well as hole trapping by electron donor ligands. Analytes bind to open surface metal coordination sites and compete with O2 for occupied metal surface sites. There is the additional possibility of weak binding (physisorption) to the organic region of the phthalocyanines molecule for noncoordinating analytes, which may be governed by weak hydrophobic and possibly charge transfer interactions. Metal-analyte coordination strength has been shown to govern analyte binding and therefore the response of CoPc chemiresistive sensors to non-oxidizing vapors. Similarly, for H2Pc chemiresistive sensors, the hydrogen bonding of analyte to the two interior NH protons has been found to primarily govern sensor responses to water and volatile organic compounds.